In video recorders it is known to record the video signal of a picture carrier on a tape along so-called oblique tracks which extend at an angle of about 6.degree. to the longitudinal direction of the tape. Preferably, one picture field is recorded along each such oblique track. Recording and playback are effected with two heads which alternatingly scan successive oblique tracks.
In such devices, a stationary head records the audio signal on, and plays it back from, a longitudinal track having a width of about 1 mm and extending parallel to the edge of the tape. Thus it is the longitudinal velocity of the tape and not the substantially higher relative velocity between the head and tape on the oblique tracks which has the controlling influence on recording and playback of the audio signal.
In practice, the longitudinal velocity of the tape is reduced to values of the order of magnitude of 2 cm/s in order to increase the total playing time of the tape. This low relative velocity between the tape and the audio head has an adverse influence on the quality of the recorded audio signal. High fidelity sound quality is practically impossible to achieve with such a low relative velocity and the recorded audio signal only has a frequency range of about 70 Hz to 7-10 kHz.
Due to the narrow width of the longitudinal track, there also results a relatively poor signal-to-noise ratio which becomes even worse if two audio signals are recorded for stereo playback since the width of each channel is then of necessity half that of the longitudinal track.
It is conceivable to record the audio signal along the oblique tracks together with the picture carrier in a manner similar to that employed for video discs. However, such a solution has not yet been successfully reduced to practice. On the one hand, the available frequency band is already utilized completely. The range from 0 to 1.3 MHz is taken up by the reduced-frequency, quadrature modulated chrominance subcarrier and the rest of the frequency band is occupied by the frequency spectrum of the modulated picture carrier. Recording of the audio carrier in the remaining narrow frequency gap between the modulated chrominance subcarrier and the frequency spectrum of the picture carrier is practically impossible because of the required steep filter band edges for the chrominance subcarrier and the picture carrier, due to the occurring phase and group delay errors.
On the other hand, if the audio signal is recorded on the oblique tracks, there results considerable interference during playback due to the switching between video heads, which does not interfere with picture playback because it takes place during the vertical blanking periods. During playback of the audio signal, however, this switching of heads at a frequency of 50 Hz becomes audible as an annoying noise component because the playback of the audio signal, which is modulated, for example, on a carrier, is temporarily interrupted due to the switching of heads. The resulting spike-type interference pulses at a fundamental frequency of 50 Hz with a large harmonics content produce a continuous, annoying noise during audio playback.